https://www.spiegel.de/international/world/nato-s-eastward-e...:
After speaking with many of those involved and examining previously classified British and German documents in detail, SPIEGEL has concluded that there was no doubt that the West did everything it could to give the Soviets the impression that NATO membership was out of the question for countries like Poland, Hungary or Czechoslovakia.
On Feb. 10, 1990, between 4 and 6:30 p.m., Genscher spoke with Shevardnadze. According to the German record of the conversation, which was only recently declassified, Genscher said: "We are aware that NATO membership for a unified Germany raises complicated questions. For us, however, one thing is certain: NATO will not expand to the east." And because the conversion revolved mainly around East Germany, Genscher added explicitly: "As far as the non-expansion of NATO is concerned, this also applies in general."
Shevardnadze replied that he believed "everything the minister (Genscher) said."
Edit: https://books.openedition.org/ceup/2759?lang=en says:
In the session with Baker on February 9, 1990, Gorbachev discussed various forms of German unification and association with NATO. Although he could not accept it publicly, he agreed with Baker’s argument that the presence of U.S. troops in Europe was a factor in overall European stability, which implicitly meant accepting the idea of Germany’s eventual membership in the Western alliance. It was during this conversation that Baker offered Gorbachev guarantees (that is the word used in the Russian memorandum of conversation—garantii) that NATO would not “spread an inch eastward,” and the Soviet leader accepted the statement as sufficient on the basis of the trust he felt had been built between him and the U.S. leadership—never asking for a written pledge. Gorbachev’s reasoning could partially be explained by the domestic dilemma he faced: how could he tell the Politburo that he had asked for written guarantees that NATO would not expand to the territories of the Warsaw Pact while the Pact was still in existence? That would have meant he had already accepted the idea that the socialist alliance was on its deathbed. In this conversation, Baker was not trying to mislead Gorbachev in any way; he was merely expressing the official position of the U.S. government at the time, which was fully shared by other Western leaders, talking specifically about the NATO presence on East German soil, but by implication also about any future expansion. However, the Bush administration would change that position very soon—without providing any notification to Gorbachev.